In a multi-antenna broadcast channel, data may be sent to more than one User Equipment (UE) simultaneously by using space resources, which is known as an SDMA technology. In contrast to the traditional Time Division Multiple Access (TDMA), the SDMA can double the system throughput. Studies show that when data is sent to multiple UEs, space resources can be allocated through Dirty Paper Code (DPC) to accomplish the best performance of the SDMA. However, such a solution is not practical because it is too complicated and does not take on a cause-result relation. Currently in an SDMA system, a common method of allocating space resources is a simpler multi-user pre-coding technology such as Zero Forcing (ZF) multi-user pre-coding technology, Minimum Mean Square Error (MMSE) multi-user pre-coding technology, or multi-user pre-coding technology based on generalized characteristic values and iterative pre-coding technology. Such technologies accomplish high performance at time of allocating space resources. If implemented together with the user scheduling technology, such technologies can achieve multi-user diversity of the broadcast channel. However, if such technologies are used together with the user scheduling technology, the Base Transceiver Station (BTS) in the SDMA system needs to know Channel State Information (CSI) of all UEs precisely beforehand. In the SDMA access system, if the CSI of all UEs are fed back, too much overhead is generated, and the solution is hardly realizable.
Therefore, the prior art puts forward a limited feedback SDMA technology, which comes in two types depending on the content of the feedback information: SCDMA based on channel quantization and SDMA based on a pre-coding version. The SDMA based on channel quantization quantizes the CSI of the UE, and generates a channel quantization codebook which is previously known to the BTS and the UE, thus reducing the quantity of the feedback information. When the requested user quantity is greater than the maximum concurrent user quantity supported by the SDMA, the pre-coding and the user scheduling generally need to be designed jointly. The SDMA based on a pre-coding codebook involves design of a pre-coding codebook or employs a random scheduling technology. The pre-coding codebook may use a random unitary matrix (U-matrix), select multiple UEs with good channel conditions according to the user feedback information, and send data concurrently through the random U-matrix. The SDMA based on a pre-coding codebook is easy to implement, involves little overhead of CSI feedback, and requires the UE to feed back only the identifier (ID) of the preferred beam of the UE and its Signal-Interference-Noise Ratio (SINR) (or the corresponding capacity). With the increase of the user quantity, the theoretic optimality and capacity growth rate can be accomplished. However, in a sparse SDMA access system, namely, in an SDMA access technology with few users, strong interference generally exists between concurrent users which are scheduled through SDMA scheduling based on a pre-coding codebook because the user quantity is low and the pre-coding matrix is generated randomly. Therefore, the enhancement of the performance of the SDMA system is rather limited.
Specifically, the SDMA based on a pre-coding codebook may also use a random U-matrix as a pre-coding codebook. In this case, the SDMA system sends data to the user in the following way. First, the UE prefers a beam ID (the number of columns of the pre-coding matrix is regarded as the beam ID) in the U-matrix, and feeds back the beam ID to the BTS; the UE uses the preferred beam and the capacity supported when interference from other beams exists (other beams and the preferred beams of the UE belong to the same pre-coding matrix). Afterward, the BTS selects the corresponding beam in the U-matrix according to the preferred beam fed back by the UE, sends the beam to the corresponding best UE (the preferred beam of the UE is directed to the best UE, and this UE supports higher capacity than other UEs with the same beam direction), and calculates out the capacity supported by the U-matrix. In this way, the BTS in the SDMA system can select the U-matrix of highest capacity as a sending matrix, and the best UE corresponding to the beam of the matrix serves as the sending UE. In this method, the whole U-matrix is selected as a sending matrix fixedly.
The foregoing solution shows that, in the limited-feedback SDMA technology, data is sent by the maximum number of concurrent UEs supported by multiple antennas of the BTS simultaneously. In order to optimize the sending performance, the concurrent data streams need to undergo power allocation similar to water injection. However, the BTS based on the limited-feedback SDMA technology lacks enough broadcast channel information, and the existing solution to sending data in an SDMA system generally supposes that the concurrent UEs send signals at equal power, which leads to loss of sending performance to some extent. Further, in a sparse SDMA system, when the user quantity is far less than the maximum number supported by multiple antennas of the BTS, if the pre-coding codebook that bears the data is sent by the maximum number of UEs supported by the multiple antennas of the BTS simultaneously, strong interference exists between the concurrent UEs that currently access the SDMA system, and the enhancement of the throughput performance of the SDMA system is rather limited.